This invention is concerned with a process for producing reinforced polymer sheet by paper-making technology, starting from an aqueous dispersion of a particulate thermoplastic polymer and reinforcing fibers having a minimum length of 2 mm.
It is known to produce fiber reinforced polymer articles of manufacture by various methods, e.g. melt-extrusion of a fiber-containing polymer composition to form sheet, film, slabs, tubes, piping, profiles etc., calendering of similar compositions to form sheet or film, injection-molding, blow-molding and compression-molding of similar compositions to form molded objects, particularly those having a reduced wall-thickness, e.g. cups, bottles and other containers. Such techniques present no particular problems provided staple fibers of relatively short length, i.e. having a length of less than 2 mm, are employed. With longer fibers very few of the above techniques are to date practiced on a truly commercial scale since there are very restricted possibilities for achieving the desired degree of homogeneity in the mixture of fiber and polymer at the moment the molding or shaping is about to be effected. Since the reinforcing effect is a function of fiber length, there exists a general incentive to employ longer fibers, e.g. having a length of at least 5 mm. The process as used by Azdel Inc., disclosed in U.S. Pat. Nos. 4,692,375 and 4,615,717, comprises impregnation of a mat of glass fiber strands with a hot molten thermoplastic resin from an extruder, and cooling the resin to form a finished fiber reinforced thermoplastic resin sheet. The product of this process is a flat composite sheet having good tensile strength in the longitudinal direction of the sheet.
It has been proposed to solve the problem related to the processing of long reinforcing fibers in thermoplastics by a completely different technology which is a paper-making process based upon using a thermoplastic polymer in particulate form e.g. a powder or granules, ground down to a very small particle size, as a binder for the fibers. Thus, a paper is produced when the fibers are cellulose fibers, or a paper-like material or synthetic paper can be produced from metal fibers, glass fibers, carbon fibers, nylon fibers, polyamid fibers, etc.
The commercial development of this different technology basically hinges around the use of olefin polymers, e.g. polyethylene, polypropylene or polybutene as the binding polymer. EP-A 6930, EP-A 100720, EP-A 180863 and FR-A 2530724). The binding performance of such polyolefins is however rather poor, most likely due to their hydrophobic properties. Typically, it is necessary to use various surfactants, detergents or flocculating agents in order to improve wettability performance of the polyolefins. These auxiliary agents may even have to be employed at different stages of the paper-making process, which significantly complicates the technology. There is thus a need to find a simplified process for making paper.
Unexpectedly, the Applicants have found that another thermoplastic polymer has a good wettability and that there is no strict need to employ the auxiliary agents referred to hereinabove. This polymer is an alternating copolymer of carbon monoxide and an olefinically unsaturated compound, with a molecular weight of at least 2000, and more preferably at least 6000, known as polyketone.
The present invention differs from the Azdel process in that it (A) comprises staple fibers instead of a mat of fiber strands, (B) dispenses an aqueous suspension of grinded polymer together with fibers onto a filter and thereafter removing the water and drying at elevated temperature the self-supporting sheet of reinforced polymer, and (C) compression molds the preform. The product of the present invention is a reinforced, optionally shaped, polymer article. Furthermore, the polymer of the present invention is an alternating copolymer of olefinically unsaturated compounds and carbon monoxide which are very suitable for this process.